refactor(dev): expose dev app and entrypoint

[ematipico]

Changes

This PR does the following:

• adds a new specifier to Astro named astro/app/dev. This new specifier exposes two bindings: a default, which is createExports, and DevApp if users need to use it
• adds a devEntrypoint to the adapters, which is used inside the Astro dev server plugin. If it is absent, the plugin uses astro/app/dev as a fallback

This should get us closer to use the cloudflare plugin

Testing

Minimal and React examples still work

Docs

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Latest commit: 80755e5

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